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form is characteristic of the genus. It is heart-shaped, more or
less, In J&alaena, and somewhat cross- or T-shaped in the genus
JZulacnnpterfi* In the Odontocetes the ribs have, some of them,
the normal attachment by capitulum and tuberculum. In the
Mystac-oeetes the at-
tachment, where it
exists, is very loose,
and the tuberculum
alone is attached to
its vertebra. This
allows of the freer play
of the ribs during re-
spiration. The scapula
has a very character- Fir"18*—SMe vie'
Lstic form in these
animals. The ucro-
luioii, where it exists,
is placed near the anterior margin of the shoulder blade, and
overlaps the generally long coracoid process. Clavicles are totally
absent. The pelvis is very rudimentary, consisting merely of a
single bonelet, to which are attached the rudiments (in some cases)
of a femur, and, in JBalaena (Fig. 188), of a tibia also.

Whales are to be divided into three great groups:—(1) the
Whalebone Whales or M ystacoeeti ; (2) the Toothed Whales or
Odontoceti ; and (3) the entirely-extinct'Archaeoceti or Zeu-

af Lones of posterior extremity of
Grtciilariii Right Whale {J&xlaena my slice t us j. x J,
i, I.*,tihium ; /» femur; f, accessory ossicle repre-
senting the tibia. (After Eschricht and I'eiuhardt)
(from Flower's Qstevltujy.)

This division is thus characterised :---Teeth are never function-
ally developed ; they are present in the young, but replaced in
the adult by the baleen or whalebone ; the external respiratory
aperture is double; the skull is perfectly symmetrical; the rami
of the mandible are arched outwards and do not form a true
symphysis; the sternum is always composed of a single piece of
bone; the ribs articulate only with the transverse processes of
the vertebrae.
The Mystacoceti are nearly invariably huge creatures, the
sole exceptions being the Pygmy Right Whale, Neobcdaena, and
x                                                                         2 A